1. Field of the Invention
The present invention relates to a semiconductor package structure, and more particularly, to a semiconductor package structure including a MOSFET and a controlling integrated circuit (IC) integrated into one package, wherein the controlling IC is attached to a lead frame using an epoxy molding compound (EMC) pad between the controlling IC and the lead frame. The present invention also relates to a method for fabricating the EMC pad.
2. BACKGROUND OF THE RELATED ART
With the trend toward smaller and lighter electronic products, efforts continue in order to reduce the number of semiconductor parts, while developing higher density mounting packages. A Switching Mode Power Supply (SMPS) method is being used to satisfy the above and other recent trends. In SMPS structures, semiconductor devices that require a large quantity of power and/or emit a large quantity of heat during the operation, i.e., a MOSFET that is used as a power transistor and a controlling IC for controlling the MOSFET, are integrated in a single package and sealed hermetically.
According to the SMPS method, first, the switching device such as the MOSFET and the controlling IC are wire bonded to each other so that they can communicate with each other. Second, the functions of peripheral devices are inserted into the controlling IC. The switching device and the controlling IC are then integrated in a single package.
In developing such a package, an insulating adhesive material is used for preventing a short between the controlling IC and a drain of the MOSFET switching device.
Various methods for enhancing the dielectric strength of the insulating adhesive material have been provided. First, a Si.sub.x N.sub.y layer, for example a Si.sub.3 N.sub.4 layer, is applied to a back side of a wafer up to a thickness of from 2 to 5 .mu.m. Generally, the layer is deposited approximately 1 .mu.m at a time. Accordingly, the number of applications can be reduced or increased depending upon the desired thickness. Thereafter, the wafer is divided into respective controlling ICs 1 as shown in FIG. 1. The divided controlling IC 1 is mounted to a portion of a die pad 5 using an insulating epoxy adhesive material 3. On another portion of the die pad 5, a MOSFET 2 is mounted using a conductive solder adhesive material 4. The IC 1 is wire bonded to the MOSFET 2 using a metallic wire 10. The IC 1 and the MOSFET 2 are bonded to inner leads of a lead frame using metallic wires (not shown). Thereafter, an encapsulation step and a trimming step complete the package.
However, it is difficult to achieve a thick layer of insulating material by deposition onto the back side of the wafer. Also, when the wafer is sawed into pieces to form individual chips, the insulating material is not present on the lateral sides of the chips. This results in a reduced dielectric strength.
As shown in FIG. 2, a controlling IC 1 and a power device 2, such as a MOSFET or a bipolar transistor, are mounted on a die pad 5 of a lead frame. The power device 2 is mounted on the die pad 5 using a conductive solder adhesive material 4 because a back side of the MOSFET is used as a drain. On the other hand, the controlling IC 1 is mounted on the die pad 5 using an insulating epoxy adhesive material 3 because the controlling IC 1 should be electrically isolated from the die pad 5. In the adhesive material for the controlling IC 1, i.e., the insulating epoxy adhesive material 3, solid insulating beads 9 may be added. The insulating beads 9 allow for precise thickness adjustment of the epoxy adhesive material 3.
However, it is difficult to find a material having a high dielectric strength at a high temperature for use as the beads. Also, it is not easy to form the beads in a spherical shape so as not to cause any voids. Furthermore, when the adhesive material including the beads is provided, semiconductor manufacturers cannot inspect for the existence of the beads in the adhesive material.
FIG. 3 is a semiconductor package using a ceramic pad. Before the controlling IC 1 is mounted on the die pad 5, a ceramic pad 6 is attached to the die pad 5 using an adhesive material 7. The ceramic pad 6 is a thin film of a ceramic material such as Al.sub.2 O.sub.3. The controlling IC 1 is insulated from the die pad 5 by the ceramic pad 6. As a result, a sufficient dielectric strength can be obtained.
However, incorporating the ceramic pad greatly increases production costs. In addition, since the ceramic pad is thin, it is easily broken by an external impact.